The present invention relates to rotary compressors and, more particularly, to such compressors of the oil-free type having mating impellers, each of which are of single-lobed construction.
The prior art is replete with varied designs of rotary, oil-free compressors. However, there is yet to be demonstrated in the operation of such compressors one which will attain a high compression efficiency while, at the same time, controlling or reducing the leakage rate. This is especially true in the one hundred pound, industrial air supply market in the popular size ranges of 100 to 200 CFM.
In general, rotary compressors can be classified as either of the screw-type or non-screw type. The screw type compressor while exhibiting good internal compression efficiency is, by nature of its three dimensional construction, a high leakage device. At the 200 CFM level, a successful screw-type compressor has yet to be demonstrated.
In an attempt to reduce leakage losses and simplify construction many two-dimensional compressor designs have been proposed. These designs will be discussed in greater detail hereinbelow. However, suffice to say, that none evidence a combination of compression efficiency and leakage control sufficient to meet the generally accepted standard of 25 horsepower per 100 CFM.
To achieve the one hundred pound output level in the size ranges of 100 to 200 CFM, two separate compression stages must be employed with appropriate intercooling therebetween. Each stage should have a compression ratio of about three. At this compression ratio it is necessary to attain an overall stage compression efficiency of 64 percent to meet accepted performance levels. If a classical Roots compressor, which does not employ internal precompression, were to be used for each stage the resulting compression efficiency would be 64.5 percent, neglecting leakage and tare losses. In as much as the ideal performance level of the Roots compressor is at the threshold of acceptability, it is instructive and convenient to utilize this simple mechanism as a comparator of performance. Some prior designs, which have employed internal precompression features to improve efficiency, have, in fact, resulted in lower efficiencies than would have been obtained with classical Roots compressors; this being due to excessive leakage and overpressure losses. Yet, to achieve the abovementioned acceptable performance levels, it is essential to employ a high degree of built-in or internal precompression. In an ideal case the gain in compressor efficiency afforded by precompression is 35.5 percentage points. Thus, the importance of precompression is abundantly clear. The goal, however, is to attain precompression in a device that can maintain good leakage control, while at the same time, not giving rise to high overpressures when the gas is discharged.
Certain prior compressors as typified by U.S. Pat. Nos. 2,097,037; 3,535,060; 3,894,822; 3,723,031; 3,790,315; and 4,224,016 result in a pressure build-up phase (precompression) followed by a separate discharge phase whereby for about one-half the time gas is discharged and for about the remainder of the time there is no gas discharge at all. These types of constructions must necessarily be limited to low impeller pitch line velocities of less than 100 feet per second in that such nonsteady discharge pulsations would result in excessive overpressure losses if the impeller velocity is high. Yet, in contrast, it is necessary to have a low ratio of leakage rate to impeller displacement rate. However, to attain such a low ratio high pitch line velocities are necessary. Thus, it would appear that these slow speed, non-steady type compressors are not capable of attaining the desired efficiency levels.
Other prior known two-dimensional compressor designs are typified by U.S. Pat. Nos. 2,266,820 and 4,076,469. These devices markedly reduce the overpressure losses and thereby permit much higher impeller velocities than could be employed in the aforementioned non-steady devices. However, the impellers on these prior devices employ at least three separate profiles, each profile having at least two lobes. These impellers are costly to manufacture, align and assemble. Further, due to the high number of mating transition surfaces on each coacting profile, the leakage losses are high. An additional problem of the last mentioned patents, which is tied to their structure, is that if three or more profiles are required for the pressure build up and delivery process there is a costly axial leakage path between the top of the lobe of the middle profile (profiles) and the compressor housing which allows the high pressure gas in the third or last profile to leak directly back to the inlet pressure gas in the first profile. With a two profile construction this costly leak path does not exist.